Heat and water vapor exchangers (also sometimes referred to as humidifiers, enthalpy exchangers, or energy recovery wheels) have been developed for a variety of applications, including building ventilation (HVAC), medical and respiratory applications, gas drying or separation, automobile ventilation, airplane ventilation, and for the humidification of fuel cell reactants for electrical power generation. When constructing various devices intended for the exchange of heat and/or water vapor between two airstreams, it is desirable to have a thin, inexpensive material which removes moisture from one of the air streams and transfers that moisture to the other air stream. In some devices, it is also desirable that heat, as well as moisture be transferred across the thickness of material such that the heat and water vapor are transferred from one stream to the other while the air and contaminants within the air are not permitted to migrate.
Planar plate-type heat and water vapor exchangers use membrane plates that are constructed using discrete pieces of a planar, water-permeable membrane (for example, Nafion®, natural cellulose, sulfonated polymers or other synthetic or natural membranes) supported by a separator material (integrated into the membrane or, alternatively, remains independent) and/or frame. The membrane plates are typically stacked, sealed, and configured to accommodate fluid streams flowing in either cross-flow or counter-flow configurations between alternate plate pairs, so that heat and water vapor is transferred via the membrane, while limiting the cross-over or cross-contamination of the fluid streams.
One well known design for constructing heat exchangers employs a rotating wheel made of an open honeycomb structure. The open passages of the honeycomb are oriented parallel with the axis of the wheel and the wheel is rotated continuously on its axis. When this concept is applied to heat exchange for building ventilation, outside air is directed to pass through one section of the wheel while inside air is directed to pass in the opposite direction through another portion of the wheel. An energy recovery wheel typically exhibits high heat and moisture transfer efficiencies, but has undesirable characteristics including a fast rotating mass inertia (1-3 seconds per revolution), a high cross-contamination rate, high pollutant and odor carryover, a higher outdoor air correction factor than is ideal, a need for an electrical energy supply to power geared drive motors, and a need for frequent maintenance of belts and pulleys. Energy recovery wheel transfer efficiency correlates to the rotational speed of the device; spinning the wheel faster typically increases the energy transfer rate. However, any efficiency gained in this manner is offset by more negative effect of the undesirable characteristics here noted. Thus there is a need for a device that exhibits an energy transfer efficiency at least as great as an energy recovery wheel while minimizing these undesirable characteristics, especially the cross-contamination.
An energy recovery wheel processes large volumes of airflow in a relatively low volume footprint. By contrast, the size of a typical cross-flow and counter-flow plate-type exchanger design increases exponentially as the volume of processed airflow increases. As a plate-type exchanger increases in size, pressure drop across the exchanger also increases. Plate spacing on large plate-type exchangers is generally increased to mitigate pressure drop. The increase in plate spacing typically increases the overall volume of the exchanger relative to its design airflow. A further disadvantage is the incompatibility of existing plate-type exchangers to fit into existing air handling units designed to accommodate the relatively thin depth profiles of energy recovery wheels prohibiting retrofit replacement of a wheel by a typical plate-type exchanger.
Energy recovery wheels are typically customized for different end-use applications. The need for customization increases the end-use cost of the exchangers, material waste during manufacturing, design time, failure-testing costs, and a number of performance verification certifications. Energy recovery wheels require a wide variety of structural support sizes, lengths, and quantities and often competing design tradeoffs including number of segments, wheel depths, motor sizes, belt lengths, and wheel speeds. In some HVAC systems, use of an energy recovery wheel may be prohibited due to the inherent risk of failure of the motor, belts, and seals.
Likewise, plate-type energy exchangers are typically customized for different end-use applications. The number and dimensions of cores are dictated by the end-use application. Manufacturing of plate-type exchangers requires the use of custom machinery, custom molds and various raw material sizes. Plate-type energy exchanger designs utilize a large number of joints and edges that need to be sealed; consequently, the manufacturing of such devices can be labor intensive as well as expensive. The durability of plate-type energy exchangers can be limited, with potential delaminating of the membrane from the frame and failure of the seals, resulting in leaks, poor performance, and cross-over contamination (leakage between streams).
In some heat and water vapor exchanger designs, the many separate membrane plates are replaced by a single membrane core made by folding a continuous strip of membrane in a concertina, zig-zag or accordion fashion, with a series of parallel alternating folds. Similarly, for heat exchangers, a continuous strip of material can be patterned with fold lines and folded along such lines to arrive at a configuration appropriate for heat exchange. By folding the membrane in this way, the number of edges that must be bonded can be greatly reduced. For example, instead of having to bond two edges per layer, it may be necessary only to bond one edge per layer because the other edge is a folded edge. However, the flow configurations that are achievable with concertina-style pleated membrane cores are limited, and there is still typically a need for substantial edge sealing, such as potting edges in a resin material. Another disadvantage is the higher pressure drop as a result of the often smaller size of the entrance and exit areas to the pleated core.
Existing cross-flow cores have theoretical efficiency limitations of approximately 80%, while the efficiency of a counter-flow core can theoretically reach 100%. Some current counterflow plate type arrangements have achieved heat transfer efficiencies equal to or greater than energy recovery wheels, but incur the penalties of a much greater volume, higher pressure drop, and higher cost when compared to a recovery wheel. A broad array of shapes have been proposed in the prior art, including long rectangles, hexagonal profiles, and back-to-back cross flow designs. The existing counter-flow plate designs utilize a greater amount of material than their related cross-flow plate exchanger counterparts. In addition, current counter-flow plate designs generally transfer thermal energy only. Counter-flow heat and moisture plate-type exchangers have been expensive to produce due to inherent difficulty of the plate separation techniques, plate sealing, and inefficient use of materials.
While an energy recovery wheel transfers heat and moisture at nearly equal efficiencies, the existing membrane-type plate-exchangers have substantially reduced moisture transfer rates in comparison to thermal energy transfer. Attempts to increase vapor transmission have employed very expensive and specialized polymeric membranes, and have not seen wide spread practical use. This is partially due to spacer materials and membrane seam bonding that are impermeable to water vapor, effectively reducing the available surface area for water transport. In addition, specialized polymeric membranes transfer water vapor substantially in only one direction, perpendicular to the planar surface. Thus, spacing techniques blocking the effective surface area of one side of the membrane inherently inhibits the vapor transmission on the opposite side of the membrane.
When adapting existing plate-type exchangers for large flow applications, a customized metal manifold system is generally employed. This customized, integrated system nearly doubles the cost of the complete assembly; further isolating it from economically competing with energy recovery wheels. Generally, the free-standing manifold system is assembled in the field requiring a significant amount of additional labor. Standard plate exchangers are often slid into pre-defined grooves resulting in a plurality of exchangers. It is difficult to ensure that the multitude of seals between the manifold system and the plate-type exchangers are properly sealed as this work is conducted on site without the proper testing instrumentation. Cross-flow exchangers employed in a typical manifold arrangement are oriented on a 45 degree angle, further increasing the overall depth of the unit making them incompatible with air handling unit designed for energy recovery wheels.